head back uptown. The Brooklyn Bridge–City Hall station contains four tracks, two island platforms, and two unused side platforms. From each island platform, one elevator and one stair lead to the station's southern overpass, two stairs lead to the northern overpass, and one stair leads to the north-end underpass. The island platforms allow for cross-platform interchanges between local and express trains heading in the same direction. Terminating trains use the outer tracks while through trains use the inner tracks. The island platforms were originally long, but became long during the 1962 renovation. The station is approximately long and wide. Platform extensions are at both ends of the original platforms. The ones at the south end are closed off, but contain gap fillers and original mosaic tiles. The 1962 platform extensions are at the north end; it was deemed easier to lengthen the express platform northward, as the curves at the south end were extremely difficult to reconstruct. There are two unused side platforms, one beside either local track. A combination of island and side platforms was also used at 14th Street–Union Square on the IRT Lexington Avenue Line and 96th Street on the IRT Broadway–Seventh Avenue Line. These side platforms were built to accommodate extra passenger volume and were built to the five-car length of the original IRT local trains. When trains were lengthened, the side platforms were deemed obsolete, and they were closed and walled off in 1910. The side platforms house electrical equipment and are blocked off with metal grates. A staircase from the western side of the mezzanine leads to the original western side platform. A sliding grate was installed on the bottom of the deck leading to the eastern side platform, which was used to prevent access to that platform when it was not in service. Design As with other stations built as part of the original IRT, the station was constructed using a cut-and-cover method. The tunnel is covered by a "U"-shaped trough that contains utility pipes and wires. The bottom of this trough contains a foundation of concrete no less than thick. Each platform consists of concrete slabs, beneath which are drainage basins. The platforms contain I-beam columns spaced every . Additional columns between the tracks, spaced every , support the jack-arched concrete station roofs. There is a gap between the trough wall and the platform walls, which are made of -thick brick covered